In English philology, AB language refers to the early Middle English

Middle English

Middle English is the stage in the history of the English language during the High and Late Middle Ages, or roughly during the four centuries between the late 11th and the late 15th century....

 dialect of Ancrene Wisse

Ancrene Wisse

Ancrene Wisse or Guide for Anchoresses is an anonymous monastic rule for anchoresses, written in the early 13th century. Ancrene Wisse was originally composed for three sisters who chose to enter the contemplative life...

and the Katherine Group

Katherine Group

The so-called Katherine Group is a group of five 13th century Middle English texts composed by an anonymous author of the English West Midlands. The texts are all addressed to anchoresses and praise the virtue of virginity....

 (13th century). The 'A' and 'B' refer to extracts in Hall's Selections from Early Middle English, 1130-1250 where 'A' is from the Katherine Group texts in MS Bodley 34 and 'B' is from Ancrene Wisse in MS Corpus Christi 402.

The term was coined in 1929 by J.R.R. Tolkien who noted that the dialect of both manuscripts is highly standardized, pointing to "a 'standard' language

Standard language

A standard language is a language variety used by a group of people in their public discourse. Alternatively, varieties become standard by undergoing a process of standardization, during which it is organized for description in grammars and dictionaries and encoded in such reference works...

 based on one in use in the West Midlands

West Midlands (region)

The West Midlands is an official region of England, covering the western half of the area traditionally known as the Midlands. It contains the second most populous British city, Birmingham, and the larger West Midlands conurbation, which includes the city of Wolverhampton and large towns of Dudley,...

in the 13th century."